Ask Dr. K: Preventing kidney stones

Dear Dr. K: I recently passed a kidney stone — and I would do anything to prevent another one from forming. Can kidney stones be prevented?

Dear Reader: I've never had a kidney stone myself, but I've had plenty of patients who suffered from them. So I know that they really hurt.

Kidney stones are hard chemical deposits that form inside the kidneys. They are often as small as grains of sand and pass painlessly out of the body in urine. But kidney stones can be much bigger — the size of a pea, a marble or even larger. If the stones just stay in the kidney and don't block the flow of urine, you won't know they are there.

But if a stone gets into the ureters — the tubes that connect each kidney to the bladder — it can cause intense pain and bleeding. This may require medication and/or hospitalization to have the stone removed or broken into fragments.

Kidney stones form if too many minerals are excreted into the urine. Low fluid volume, caused by dehydration, is another cause. Most kidney stones are made of calcium oxalate, but stones may be made of other substances. Your doctor can determine what your stone is made of by analyzing your urine or the kidney stone itself.

People who have already had a kidney stone have the highest risk of forming another one. Fortunately, kidney stones can be largely prevented with diet and lifestyle changes and, sometimes, medication. Your doctor will tailor recommendations to what your kidney stone is made of.

Prevention may involve:

· Water. Drinking plenty of fluids keeps your urine dilute, so minerals won't accumulate into stones. Drink at least 10 glasses of fluid a day, half of which should be water.

· Calcium. Most kidney stones contain calcium. Surprisingly, dietary calcium is protective. That's because it reduces the amount of oxalate that is absorbed from the gut, and it is the combination of calcium and oxalate that forms stones. Good sources include milk, cheese, yogurt, oranges and broccoli. On the other hand, calcium supplements can increase your risk for kidney stones because they increase the calcium in your blood and urine but do not decrease the oxalate. Avoid calcium supplements if you've had calcium kidney stones.

· Oxalate. Oxalate often binds to calcium to form kidney stones. Cut down on high-oxalate foods, which include beets, spinach, chard, rhubarb, tea, coffee, cola, chocolate and nuts.

· Sodium. Keep sodium intake under 1,500 mg a day. A low-sodium diet reduces the amount of calcium the kidney puts into the urine.

· Protein. Avoid or limit red meat. Animal protein increases the tendency of stones to form in the urine.

· Fruits and vegetables. Plant-based foods protect against kidney stones. High-potassium fruits such as tomatoes and bananas are best. Citrus fruits — except grapefruit — are another good choice.

· Obesity. Obesity increases risk. Shed excess pounds.

· Medication. If needed, your doctor can recommend medications based on the composition of your stones.

Dr. Komaroff is a physician and professor at Harvard Medical School. See his website to send questions and get additional information: www.AskDoctorK.com.